1. Field of the Invention
The present invention relates to a progressive power (multi-focal) lens for eyeglasses, in which the surface power continuously changes from a distance portion to a near portion. In particular, the present invention relates to a progressive power lens which is provided on a peripheral edge thereof with a rim surface portion whose width is reduced to provide an increased effective surface area of the lens. The present invention also relates to a mold which produce such a progressive power lens.
2. Description of Related Art
In a known process of producing a plastic lens for eyeglasses, a monomeric material 54 in liquid state is introduced and heated in a cavity defined between molds 51, 52 and a gasket 53 to polymerize the same, so that a lens 50 of a solid polymer can be obtained, as shown in FIG. 16.
The gasket 53 is made of a relatively elastically deformable material to confine the monomer 54 in the mold cavity defined between the molds 51 and 52 and to absorb a change in volume due to the polymerization from a monomer to a polymer.
The mold to produce a plastic single focal lens or a plastic multi-focus lens having a distance portion and a near portion separate from the distance portion, or a semi-product thereof, is usually provided with a spherical mold surface, and hence, the thickness T of the gasket 53 to be used is constant over the entire periphery of the lens. However, for a mold to produce an astigmatic power lens as a final product, a gasket whose thickness varies in accordance with a shape of a toric lens surface must be used. To reduce the number of kinds of gaskets to be prepared, the toric surfaces are systematized.
In the mold for the progressive power lens, the shape of the surface thereof on the progressive surface (progressive side) is a complex aspherical surface. Moreover, for example, in the arrangement of the mold and the gaskets as shown in FIG. 16, the thickness of the gaskets are not uniform. In particular, in a mold for the progressive power lens whose optical center is deviated from the center of the circle (diameter) of the lens, the change in the thickness of the gasket, along the outer periphery thereof, is much more complicated.
Assuming that, in coordinate systems as shown in FIGS. 17 and 18 in which the diameters of the lens 50 and the molds 51, 52 are indicated by 55 and 56, respectively, and coordinates Z1 and Z2 (FIG. 18) of both surfaces of the lens 50 at an optional angle xcex8 are represented by the Z-axis, a graph as shown in FIG. 19 is obtained. The distance between Z1 and Z2, at an optional angle xcex8, corresponds to the thickness T of the gasket at the angle xcex8.
A progressive power lens includes a base curve representing the power of the distance portion and the power of the near portion in combination (note that a difference between the power of the distance portion and the power of the near portion is referred to as xe2x80x9caddition powerxe2x80x9d). There are more than 100 combinations of the base curve and the power of the near portion for one lens system (product). It is therefore impractical to prepare many gaskets corresponding to these combinations in view of the production cost or stocking thereof.
To this end, the mold 51, which forms the progressive surface of the lens, is ground flat at the peripheral portion, so that the rim surface portion (flat surface portion or non-progressive focus area) 58, which does not serve as a progressive surface (effective surface), is formed at the periphery of the lens, as shown in FIG. 20. Consequently, the gasket has a uniform thickness regardless of the angle xcex8.
Thus, the thickness T of the gasket is uniformly constant over the entire periphery of the lens as seen in FIG. 21, and the gasket can be commonly used with various kinds of progressive surfaces having different base curves and different addition powers in combination.
FIG. 22 shows a mold used to form the progressive surface (effective surface) of a progressive power lens, using the gasket having the rim surface 58. As can be seen in FIG. 22, the rim surface 58 is formed around (on upper and lower sides of) the progressive surface (effective surface) 57. The diameter of the effective progressive surface 57 is reduced in the vertical direction due to the presence of the rim surface portion 58, but no serious problem is practically caused by the reduction of the effective diameter as, in general, the diameter of a frame for eyeglasses in the vertical direction is smaller than the diameter thereof in the horizontal direction.
However, to respond to the requirement to make the eyeglasses thinner and lighter, the radius of the base curve of the progressive power lens has recently been increased. In particular, in the lens having a large negative power, if the flat rim surface 58 is provided, the reduction of the effective diameter in the vertical direction is not acceptable.
FIG. 23 shows an example of a known progressive power lens which is used to correct a highly myopic presbyopia, wherein the addition power is about 3.00 D (diopter), and the surface power of the distance portion is around xe2x88x927.00 D to xe2x88x9210.00 D (diopter). The effective progressive surface spreads over the whole lens diameter (=70 "PHgr"mm) in the horizontal direction, but is reduced in the vertical direction by the rim surface portions 58 each having a width of 8.2 mm, which are formed on the upper and lower sides of the progressive surface 57. The average surface power at the distance reference point 59 is 0.12 D and the refractive index of the blank material is 1.6. The distance reference point refers to a point on the front surface of the lens at which the corrective power for the distance portion shall apply.
FIG. 25 shows vertical and horizontal sections 60 and 61 which define the progressive surface portion 57 of a progressive power lens in an overlapped state. For clarity, in the drawings, the dimension in the longitudinal direction, only, is enlarged. The vertical and horizontal section lines of the rim surface 58 are indicated by dotted and dashed lines. The intersecting points of the progressive surface and the rim surface in the vertical section are indicated at 64 and 64xe2x80x2, and the corresponding intersecting points in the horizontal section are indicated by 65 and 65xe2x80x2, respectively. The symbol xcfx86 designates the angle defined by lines normal to the rim surface and the progressive surface. The parenthesized numerals represent the angle xcex8 in FIG. 17. xe2x80x9cWaxe2x80x9d designates the width of the upper rim surface portion, and xe2x80x9cWbxe2x80x9d the width of the lower rim surface portion, respectively. The progressive surface is inclined at an appropriate angle so that the width Wa is identical to the width Wb.
FIG. 24 shows another example of a known progressive power lens which is used to correct a myopic presbyopia, wherein the addition power is about 3.00 D (diopter), and the surface power of the distance portion is around xe2x88x923.00 D to xe2x88x926.00 D (diopter). The effective progressive surface spreads over the whole lens diameter (=75 "PHgr"mm) in the horizontal direction but is reduced in the vertical direction by the rim surface portions 58 each having a width of 4.1 mm, which are formed on the upper and lower sides of the progressive surface 57. The average surface power at the distance reference point 59 is 2.04 D and the refractive index of the blank material is 1.6.
FIG. 26, which corresponds to FIG. 25, shows vertical and horizontal sections 60 and 61 which define the progressive surface portion 57 of a progressive power lens in an overlapped state. FIGS. 25 and 26 show that the widths Wa and Wb of the upper and lower rim surface portions increase as the curvature of the base curve decreases, so long as the addition power is identical.
In the first example of a known progressive power lens, in which the curvature of the base curve is small, the angle xcfx86, defined by the progressive surface and the rim surface at the boundary between the progressive surface 57 and the rim surface 58, as shown in FIG. 25, is small on average. Consequently, the widths Wa and Wb, of the rim surfaces that correspond to the difference in the position between the intersecting points 64 and 64xe2x80x2 (in the vertical section) and the intersecting points 65 and 65xe2x80x2 (in the horizontal section) are increased.
In the two examples of prior art mentioned above, assuming that the angle xcfx86 is graphed as a function of the angle xcex8, the graphs of examples 1 and 2 are shown in FIGS. 27 and 28, respectively. The mean value AVG(xcfx86) of the angle xcfx86 and the standard deviation STD(xcfx86) are defined as follows:                               AVG          ⁡                      (            φ            )                          =                              ∫            0            360                    ⁢                                    φ              ⁡                              (                θ                )                                      ⁢                                          ⅆ                θ                            /              360                                                                        STD          ⁡                      (            φ            )                          =                              [                                          ∫                0                360                            ⁢                                                                    {                                                                  φ                        ⁡                                                  (                          θ                          )                                                                    -                                              AVG                        ⁡                                                  (                          φ                          )                                                                                      }                                    2                                ⁢                                                      ⅆ                    θ                                    /                  360                                                      ]                                1            2                              
The ratio between AVG(xcfx86) and STD(xcfx86) is an index (measure) which represents the magnitude of the change in the angle defined between the progressive surface and the rim surface. The index is 0.28 in the first example and 0.13 in the second example, respectively.
The primary object of the present invention is to provide a progressive power lens in which the width of the rim surface can be decreased even when the curvature of the base curve is small.
Another object of the p resent invention is to provide a mold to produce such a progressive power lens.
Namely, according to the basic concept of the present invention, the angle xcfx86, defined between the progressive surface and the rim surface, is set to be a relatively large value to lower the ratio of STD(xcfx86)/AVG(xcfx86), to reduce the width of the rim surface. Since the shape of the progressive surface is determined in accordance with requirements of optical performance and ornamental appearance, the rim surface, which is a planar surface in the prior art, is a curved surface in the present invention. The curved surface can be a conical surface, a toric surface, a spherical surface, a cylindrical surface, or a toroidal surface, (such as a doughnut-shaped surface), etc. The curved surface is oriented to project in the direction opposite to the projecting direction of the progressive surface.
According to the present invention, there is provided a progressive power lens having an effective surface, including a progressive surface portion which progressively varies the power, and a peripheral rim surface, which does not function as an effective surface, and which is provided to surround the effective surface. The rim surface portion is a curved surface.
In an embodiment of the invention, a progressive power lens satisfies the following relationship:
Dfxe2x89xa63xe2x80x83xe2x80x83(1)
STD(xcfx86)/AVG(xcfx86)xe2x89xa60.15xe2x80x83xe2x80x83(2)
with
STD(xcfx86) represents the standard deviation of xcfx86 over the entire circumferential length of the lens;
AVG(xcfx86) represents a mean value of xcfx86 over the entire circumferential length of the lens;
Df (diopter) represents an average surface power at a distance reference point of the progressive surface portion; and,
xcfx86 (degree) stands for the angle defined by the progressive surface portion and the rim surface portion at a boundary therebetween.
Preferably, the progressive power lens further satisfies the following relationships:
Dfxe2x89xa62xe2x80x83xe2x80x83(3)
STD(xcfx86)/AVG(xcfx86)xe2x89xa60.1xe2x80x83xe2x80x83(4)
If part of the rim surface is a spherical surface, the spherical surface is oriented to project in the direction opposite to the projecting direction of the progressive surface portion.
In other words, the surface power Ds (diopter) of the spherical surface and the average surface power Df (diopter) at the distance reference point of the progressive surface portion have different signs. Preferably, the progressive power lens satisfies the following relationships:
Dfxe2x89xa63xe2x80x83xe2x80x83(5)
Dsxe2x89xa6Dfxe2x88x922xe2x80x83xe2x80x83(6)
More preferably, the lens meets the following relationships:
Dfxe2x89xa62xe2x80x83xe2x80x83(7)
Dsxe2x89xa6Dfxe2x88x923xe2x80x83xe2x80x83(8)
A part of the rim surface can be made of a toric surface or cylindrical surface. Assuming that the surface powers of the toric or cylindrical surface in the vertical direction and in the horizontal direction are Dv (diopter) and Dh (diopter), respectively, it is preferable that Dv is equal to or greater than Dh, i.e.,
Dhxe2x89xa6Dvxe2x80x83xe2x80x83(9)
If a part of the rim surface is made of a toric surface or a cylindrical surface, the cylindrical surface is oriented to project in the direction opposite to the projecting direction of the progressive surface portion. Preferably, the progressive power lens satisfies the following relationships:
Dfxe2x89xa63xe2x80x83xe2x80x83(10)
(Dh+Dv)/2xe2x89xa6Dfxe2x88x922xe2x80x83xe2x80x83(11)
A part of the rim surface can be a toroidal surface, including a doughnut-shaped surface.
If a part of the rim surface is a conical surface, the apex thereof is oriented in the direction opposite to the projecting direction of the progressive surface portion. In this case, the progressive power lens preferably satisfies the following relationship:
Dfxe2x89xa63xe2x80x83xe2x80x83(12)
xcexa9xe2x89xa6170xe2x80x83xe2x80x83(13)
with
Df (diopter) represents an average surface power at a distance reference point of the progressive surface portion; and,
xcexa9 (degree) represents an apex angle of the conical surface.
The requirement of Dfxe2x89xa63 in the formulae (1), (5), (10) and (12) refers to the application of the present invention to a progressive power lens in which the curvature of the base curve is relatively small. If the curvature of the base curve is larger, the width of the rim surface is originally so small that no improvement is necessary.
If the lens meets the requirements defined by the formulae (2), (6), (11) and (13), a reduction in the width of the rim surface can be expected to some extent. However, if the lens does not meet these formulae, no reduction in the width of the rim surface can be obtained.
Formulae (3), (7) and (4), (8) are directed to further increase the value of the angle xcfx86 in order to satisfactorily reduce the width of the rim surface, for the progressive surface whose curvature of the base curve is smaller. However, if the angle xcfx86 is too large, the workability of the lens is remarkably worsened, and a prism error can be easily caused. Consequently, the angle xcfx86 should not be too large.
According to another aspect of the present invention, there is also provided a mold which is used to produce a progressive power lens. A progressive surface forming portion forms a progressive surface portion of the lens. A rim surface forming portion forms a rim surface portion of the lens, (which does not function as a progressive surface). The rim surface forming portion is a curved surface. The above-mentioned requirements can be equally applied to the curved surface.
The present disclosure relates to subject matter contained in Japanese Patent Application No.06-197019 (filed on Aug. 22, 1994) which is expressly incorporated herein by reference in its entirety.